spring beans源码解读之--XmlBeanFactory

时间:2021-07-10 01:56:35

导读:

XmlBeanFactory继承自DefaultListableBeanFactory,扩展了从xml文档中读取bean definition的能力。从本质上讲,XmlBeanFactory等同于DefaultListableBeanFactory+XmlBeanDefinitionReader ,如果有更好的需求,可以考虑使用DefaultListableBeanFactory+XmlBeanDefinitionReader方案,因为该方案可以从多个xml文件读取资源,并且在解析xml上具有更灵活的可配置性。

1 XmlBeanFactory的使用

典型构造方法:

    public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
super(parentBeanFactory);
this.reader.loadBeanDefinitions(resource);
}

其中,

reader是XmlBeanDefinitionReader的实例,XmlBeanDefinitionReader继承自AbstractBeanDefinitionReader(http://www.cnblogs.com/davidwang456/p/4190428.html 已经介绍过)。

Resource 接口封装了各种可能的资源类型,也就是对使用者来说屏蔽了文件类型的不同。这样所有的资源都被可以通过 InputStream 这个类来获取,所以也屏蔽了资源的提供者。另外还有一个问题就是加载资源的问题,也就是资源的加载者要统一,从上图中可以看出这个任务是由 ResourceLoader 接口完成,他屏蔽了所有的资源加载者的差异,只需要实现这个接口就可以加载所有的资源,他的默认实现是 DefaultResourceLoader。

spring beans源码解读之--XmlBeanFactory

1.1 Resource 接口继承了 InputStreamSource 接口,这个接口中有个 getInputStream 方法,返回的是 InputStream 类。

public interface InputStreamSource {
InputStream getInputStream() throws IOException;
}
getInputStream设计希望每次请求都返回一个新的流。这种要求在一个场合非常重要,例如javamail api,它在创建多个邮件附件时需要多次读取到流,在这中情况下,它就要求每个getInputStream请求能返回一个新的流。

1.2 抽象资源类AbstractResource继承自Source接口,实现了部分方法。
exists方法检查一个文件或者输入量是否可以打开;
isOpen 通常返回false;
  getUrl和getFile 抛出异常;
toString返回描述信息。 另外有一个有用的方法:lastModified()返回文件的最后修改时间。调用了File.lastModified() 返回此抽象路径名表示的文件最后一次被修改的时间。
    @Override
public long lastModified() throws IOException {
long lastModified = getFileForLastModifiedCheck().lastModified();
if (lastModified == 0L) {
throw new FileNotFoundException(getDescription() +
" cannot be resolved in the file system for resolving its last-modified timestamp");
}
return lastModified;
}

1.3 Resource的实现子类:

AbstractResource的直接子类有:AbstractFileResolvingResource, ByteArrayResource,BeanDefinitionResource, DescriptiveResource, FileSystemResource, InputStreamResource, PathResource, VfsResource。
其中,AbstractFileResolvingResource的直接子类有:ClassPathResource, PortletContextResource, ServletContextResource, UrlResource
比较重要的实现类为实现了ContextResource接口的资源类,ContextResource接口继承了Resource接口,通过从一个封装的context中加载资源来扩展Resource接口的功能。例如:可以从ServletContext,PortletContext,普通的classpth路径或者文件系统的相对路径(特别是没有指明前缀,因此适用于本地ResourceLoader的context),其子类实现有:PortletContextResource, ServletContextResource,ClassPathContextResource,ClassRelativeContextResource,FileSystemContextResource.

1.4 资源的加载ResourceLoader,ResourceLoader是一个加载资源的策略接口,可以从classpth或者文件系统中,ApplicationContext需要加载资源功能,使用了其扩展类ResourcePatternReolver。DefaultResourceLoader是该接口的标准实现,适用于一个ApplicationContext外部,可以用在ResourceEditor中。在application允许过程中,使用特定的context子原因加载策略,可以将type资源和资源组通过string的形式设置bean的属性。

spring beans源码解读之--XmlBeanFactory

(来源:http://www.ibm.com/developerworks/cn/java/j-lo-spring-principle/)

2. Bean的解析与注册详细过程分析

spring beans源码解读之--XmlBeanFactory

    (来源:http://www.ibm.com/developerworks/cn/java/j-lo-spring-principle/)

首先, XmlBeanFactory构造函数this.reader.loadBeanDefinitions(resource);

  1. XmlBeanDefinitionReader加载bean定义:loadBeanDefinitions方法

2. XmlBeanDefinitionReader解析bean定义,并注册:doLoadBeanDefinitions方法

Document doc = doLoadDocument(inputSource, resource);
return registerBeanDefinitions(doc, resource);

3.1. 使用标准的JAXP配置的xml解析器从Resource中加载到Document。

    /**
* Load the {@link Document} at the supplied {@link InputSource} using the standard JAXP-configured
* XML parser.
*/
@Override
public Document loadDocument(InputSource inputSource, EntityResolver entityResolver,
ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception { DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware);
if (logger.isDebugEnabled()) {
logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]");
}
DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler);
return builder.parse(inputSource);
}

3.2.根据加载的Document 注册Bean definition。

    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
documentReader.setEnvironment(getEnvironment());
int countBefore = getRegistry().getBeanDefinitionCount();
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}

3.2.1 创建BeanDefinitionDocument的解析器BeanUtils.instantiateClass(this.documentReaderClass)

3.2.2 BeanDefinitionDocument解析器注册bean definition,从根节点<beans>开始

    /**
* Register each bean definition within the given root {@code <beans/>} element.
*/
protected void doRegisterBeanDefinitions(Element root) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!getEnvironment().acceptsProfiles(specifiedProfiles)) {
return;
}
} // Any nested <beans> elements will cause recursion in this method. In
// order to propagate and preserve <beans> default-* attributes correctly,
// keep track of the current (parent) delegate, which may be null. Create
// the new (child) delegate with a reference to the parent for fallback purposes,
// then ultimately reset this.delegate back to its original (parent) reference.
// this behavior emulates a stack of delegates without actually necessitating one.
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createDelegate(this.readerContext, root, parent); preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root); this.delegate = parent;
}
/**
* Parse the elements at the root level in the document:
* "import", "alias", "bean".
* @param root the DOM root element of the document
*/
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element) {
Element ele = (Element) node;
if (delegate.isDefaultNamespace(ele)) {
parseDefaultElement(ele, delegate);
}
else {
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
} private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}

4. documentReader.registerBeanDefinitions(doc, createReaderContext(resource));--->

    public XmlReaderContext createReaderContext(Resource resource) {
return new XmlReaderContext(resource, this.problemReporter, this.eventListener,
this.sourceExtractor, this, getNamespaceHandlerResolver());
}

5.调用:

    protected NamespaceHandlerResolver createDefaultNamespaceHandlerResolver() {
return new DefaultNamespaceHandlerResolver(getResourceLoader().getClassLoader());
}

6.BeanDefinitionReaderUtils.registerBeanDefinition

    /**
* Process the given bean element, parsing the bean definition
* and registering it with the registry.
*/
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// Send registration event.
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}

7.处理给定的Bean元素,并解析到BeanDefinition。

    /**
* Process the given bean element, parsing the bean definition
* and registering it with the registry.
*/
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// Send registration event.
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}

8.

    public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException { // Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition()); // Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String aliase : aliases) {
registry.registerAlias(beanName, aliase);
}
}
}

9.将解析的Bean定义放入BeanDefinitionNames和BeanDefinitionMap保存已经aliasMap等

    @Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException { Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null"); if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
} BeanDefinition oldBeanDefinition; synchronized (this.beanDefinitionMap) {
oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!this.allowBeanDefinitionOverriding) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (this.logger.isWarnEnabled()) {
this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
" with a framework-generated bean definition ': replacing [" +
oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
else {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
}
else {
this.beanDefinitionNames.add(beanName);
this.frozenBeanDefinitionNames = null;
}
this.beanDefinitionMap.put(beanName, beanDefinition);
} if (oldBeanDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}

3. 一个非常重要的部分就是建立 Bean 对象实例之间的关系,这也是 Spring 框架的核心竞争力,何时、如何建立他们之间的关系请看下面的时序图:

spring beans源码解读之--XmlBeanFactory

来源:http://www.ibm.com/developerworks/cn/java/j-lo-spring-principle/

4. 小结:

  从spring 3.1版本后,xmlBeanFactory被表明为Deprecated. 推荐使用DefaultListableBeanFactory和XmlBeanDefinitionReader替换。

本文从xmlBeanFactory扩展到XmlBeanDefinitionReader,再到Resource接口。

一些情节还没有理顺,显得有些杂乱,后续会修改(写的很累,休整一下)。写这篇博客的时候,这句话一直在我脑边回想:

"写书就像冒险。一开始它是玩具和娱乐。然后她成了你的情妇。然后是你的主人。然后变成一个暴君。最后你终于认命的时候,它死了,然后给拖到外面游街。" ----温斯顿.丘吉尔

参考文献:

1. http://www.ibm.com/developerworks/cn/java/j-lo-spring-principle/